The present invention is relative to an electrical generation system, in particular to a membrane fuel cell system, particularly suitable for being employed in mobile application, for instance in vehicular applications.
Among the known types of fuel cell, membrane fuel cells are the most suited to the vehicular applications, because of their particularly simplified internal structure and of their quick start-up allowing them to reach the nominal maximum power in very short times, associated with an excellent response to sudden power peak requirements.
Beside these positive features, membrane fuel cells present however a few inconveniences: among these, particularly relevant is the need of maintaining in a fully hydrated state the proton-exchange membrane, which acts as a solid electrolyte and which commonly consists of a polymeric backbone whereon protonated functional groups, generally sulphonic, are inserted, whose dissociation, determining the proton conductivity, is in fact a function of the water content.
The water content of the membrane is the outcome of a delicate equilibrium between water produced in operation and water extracted by the gases flowing across the fuel cell. The water extraction may become dangerously high when the fuel cell is operated at moderate pressures as required to minimise the energy parasite consumption negatively affecting the global system efficiency. At moderate, and in particular at near-atmospheric operating pressures, the volumetric gas flow-rates are high; on the air side the situation is then particularly critical, since in order to maintain a sufficient oxygen partial pressure even in the cell region close to the outlet, air is supplied in a substantially higher amount, typically double, than the theoretically required value.
In the prior art, the necessary hydration of the membrane is typically preserved by external reactant humidifying systems, nevertheless the associated devices always entail a remarkable system complication and a weight and bulk increase, hardy acceptable especially in the case of mobile applications.
Also the systems for the recovery of product water from the exhausts (especially from the cathodic air) usually entail a condensation step which may be rather onerous, especially because it involves the thermal exchange with water which has to be kept at a very low temperature (indicatively 20° C., for a better efficiency) for being accepted in mobile systems which must be able to operate in any kind of environmental conditions.
A potentially interesting system to simultaneously improve the water and thermal management of fuel cell systems is disclosed in U.S. Pat. No. 6,406,807, wherein the direct injection of water inside the fuel cells is described: the evaporation effectively withdraws the heat generated by the operation simultaneously producing the steam partial pressure required for maintaining a correct membrane hydration. The method nevertheless presents a remarkable difficulty in the adjustment, also in consideration of the fact that, whereas an insufficient water supply would cause a drop in the membrane conductivity, an excessive supply would lead to the flooding of the porous electrodes and to the consequent impossibility of effectively feeding the reactants to the reaction sites, resulting in a downright performance downfall.
In small-size electrical generation systems, especially for vehicular applications, it is desirable however to make use of the sole product water to keep the water balance, also in view of the need of keeping a strict water quality control, since the smallest traces of contaminants, especially foreign cations, could get permanently bonded to the functional groups, thereby reducing the membrane conductivity.
An interesting system in this regard is the one described in WO2004/088768, according to which the fuel cells are fed with dry gases and kept hydrated by the sole product water in a significant range of operation conditions. The system is limited however by the fact that air has to be feed at a temperature below 35° C., possibly comprised between 23 and 27° C.; to get hold of such a cool air in any climatic condition, the system also comprises an ambient air compression, cooling and re-expansion cycle that also involves a certain energy expense; besides that, the system is rather complex in terms of regulations, having to control the temperature of the cathodic exhaust as a function of the air-feed pressure in order to close the balance, and especially having to instantly vary all the different process parameters according to the current density, which may suddenly vary according to the power request, especially in mobile systems.